feat(email): use Nxtgauge logo image instead of text logo

- Replace text 'NXTGAUGE' with actual logo image in email header
- Use hosted logo URL: https://nxtgauge.com/nxtgauge-logo.png
- Copy logo to email/public directory for future use

crates/email/templates/base.html

@@ -289,9 +289,13 @@
           <div class="email-wrapper">
             <!-- Header -->
             <div class="email-header">
-              <a href="https://nxtgauge.com" class="logo"
-                >NXT<span>GAUGE</span></a
-              >
+              <a href="https://nxtgauge.com">
+                <img
+                  src="https://nxtgauge.com/nxtgauge-logo.png"
+                  alt="Nxtgauge"
+                  style="height: 50px; width: auto; border: 0"
+                />
+              </a>
             </div>
 
             <!-- Dynamic Content -->